CVE-2026-104850 MCP TypeScript SDK: OAuth client could send credentials to an authorization server chosen by the MCP server
A malicious MCP server can trick the TypeScript SDK client into sending OAuth credentials to an attacker-controlled authorization server, exposing secrets.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-104850?
CVE-2026-104850 is a high-severity vulnerability in modelcontextprotocol typescript-sdk, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
MCP TypeScript SDK is the official TypeScript SDK for Model Context Protocol servers and clients. Starting in version 1.12.0 and prior to versions 1.31.0 and 2.2.0, the SDK's OAuth client support let the MCP server a client connected to decide which authorization server received the client's OAuth credentials. Stored and pre-provisioned credentials were not bound to the authorization server they belong to. A malicious or compromised MCP server could name its own authorization server in its protected resource metadata. Without any user interaction, the client would send that server the `refresh_token` and `client_secret` stored from an earlier sign-in (1.x), or the configured `client_secret` or signed assertion of a bundled non-interactive provider (1.x and 2.x). Only those applications that use the SDK as an MCP client over HTTP with an `authProvider`: your own `OAuthClientProvider`, or the bundled `ClientCredentialsProvider`, `PrivateKeyJwtProvider`, `StaticPrivateKeyJwtProvider` or (2.x) `CrossAppAccessProvider` and that may connect to an MCP server the owners does not fully trust while holding credentials for a legitimate authorization server are affected. `@modelcontextprotocol/sdk` 1.31.0 (1.x) and `@modelcontextprotocol/client` 2.2.0 (2.x) patch the issue. A workaround for those who cannot upgrade is available. 2.0.0 and 2.1.0 already accept `expectedIssuer`. On 1.x, the only workaround is to connect OAuth-enabled clients only to MCP servers you trust.
Which versions are affected by CVE-2026-104850?
4 open source packages ship an affected version, including @modelcontextprotocol/client and @modelcontextprotocol/sdk.
| Package | Affected | Fixed in |
|---|---|---|
@modelcontextprotocol/client npm | From 2.0.0 | 2.2.0 |
@modelcontextprotocol/client npm | From 2.0.0 | 2.2.0 |
@modelcontextprotocol/sdk npm | From 1.12.0 | 1.31.0 |
@modelcontextprotocol/sdk npm | From 1.12.0 | 1.31.0 |
How do I fix CVE-2026-104850?
Upgrade @modelcontextprotocol/client to 2.2.0 or later.
Is CVE-2026-104850 being exploited?
There is no known exploitation of CVE-2026-104850 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-LO03281, CLEANSTART-2026-YP03178, CLSA-2026-1791626582, CLSA-2026-1791679180, CLSA-2026-1791835900, CLSA-2026-1791839313 and 18 more.